The present invention relates to remote sensing, tracking, and identification, and in particular to the production and use of inexpensive ID xe2x80x9ctags.xe2x80x9d
Various monitoring technologies are known and used to monitor the location of an article or to provide identification in a wide range of contexts. One such technology, known as xe2x80x9ctagging,xe2x80x9d is commonly employed, for example, in shoplifting security systems, security-badge access systems and automatic sorting of clothes by commercial laundry services. Conventional tagging systems may use some form of radio-frequency identification (RF-ID). In such systems, RF-ID tags and a tag reader (or base station) are separated by a small distance to facilitate near-field electromagnetic coupling therebetween. Far-field radio tag devices are also known and used for tagging objects at larger distances (far-field meaning that the sensing distance is long as compared to the wavelength and size of the antenna involved).
The near-field coupling between the RF-ID tag and the tag reader is used to supply power to the RF-ID tag (so that the RF-ID tag does not require a local power source) and to communicate information to the tag reader via changes in the value of the tag""s impedance; in particular, the impedance directly determines the reflected power signal received by the reader. The RF-ID tag incorporates an active switch, packaged as a small electronic chip, for encoding the information in the RF-ID tag and communicating this information via an impedance switching pattern. As a result, the RF-ID tag is not necessarily required to generate any transmitted signal.
Even though RF-ID tags have only a small and simple electronic chip, they are relatively complex devices requiring sophisticated manufacturing techniques to produce. A simpler alternative involves marker elements adapted to affect an interrogation signal in a measurable, characteristic way. Many such systems utilize magnetic or magnetomechanical tags. For example, a magnetic wire or strip exhibiting harmonic behavior may be stimulated within an interrogation zone by transmitter antenna coils. The coils generate an alternating magnetic interrogation field, which drives the marker into and out of saturation, thereby disturbing the interrogation field and producing alternating magnetic fields at frequencies that represent harmonics of the interrogation frequency. The harmonics are detected by receiver antenna coils, which may be housed in the same structure as the transmitter coils. Accordingly, the appearance of a tagged article within the zonexe2x80x94which may be defined, for example, near the doors of a retail store or libraryxe2x80x94is readily detected.
While inexpensive, magnetic antitheft systems tend to encode very little, if any, information. Essentially, the tag merely makes its presence known. Although some efforts toward enhancing the information-bearing capacity of magnetic tags have been made-see, e.g., U.S. Pat. Nos. 5,821,859; 4,484,184; and 5,729,201, which disclose tags capable of encoding multiple bits of dataxe2x80x94the tags themselves tend to be complex and therefore expensive to produce, and may require special detection arrangements that limit the interrogation range (the ""859 patent, for example, requires scanning a pickup over the tag) or involve specialized equipment.
In U.S. Ser. No. 09/617,249 (filed on Jul. 14, 2000), commonly owned with the present application and incorporated by reference herein, we disclosed tags having information-encoding spatial inhomogeneities that may be detected in the time domain; in effect, characteristics in space are transformed into time for sensing purposes. We have also found that spatial homogeneities can be detected and resolved in the frequency domain. This xe2x80x9cspace-into-frequencyxe2x80x9d approach can be preferable, for example, when simultaneously interrogating multiple tags, and may also afford less implementational complexity (since circuit timing is not critical, and fabrication techniques for the tags themselves is straightforward). In any case, the present invention represents an alternative approach to remotely deriving information that has been spatially encoded.
Tags in accordance with the present invention may be very inexpensively produced yet carry appreciable quantities of data. Unlike the prior art, which requires specialized information-bearing structures, the present invention can utilize simple physical modifications to, or externally applied field biases operating on, materials that are easily procured.
In general, the present invention utilizes structures, preferably small in overall dimension, that exhibit multiple resonances at frequencies conveniently detectable through wireless broadcast. Thus, in one aspect, the invention facilitates sensing of information using an element responsive to a wireless electromagnetic signal. The element may have a plurality of non-equivalent current pathways, each of which responds differently to the signal and collectively represent the information. The element is subjected to the wireless electromagnetic signal, stimulating the current pathways, each of which contributes to an overall element response. The individual contributions and, hence, the information may be recovered from this overall response.
The response of each of the pathways to the signal may vary in terms of, e.g., one or more of resonant frequency, amplitude, damping, and Q factor. For example, each of the pathways may correspond to a different capacitance and/or to a different inductance.
It should be stressed that frequency response has been employed in prior systems to facilitate tag detection, but in a manner very different from that described herein. For example, prior-art surveillance systems based on magnetoelastic materials utilize only the fundamental mechanical resonance frequency of the marker. A representative marker includes one or more strips of a magnetoelastic material packaged with a magnetically harder ferromagnet (i.e., one with a higher coercivity) that provides a biasing field to establish peak magnetomechanical coupling. The mechanical resonance frequency of the marker is dictated essentially by the length of the strip(s) and the biasing field strength. When subjected to an interrogating signal tuned to this resonant frequency, the marker responds with a large signal field that is detected by a receiver. The size of the signal field is partially attributable to an enhanced magnetric permeability of the marker material at the resonance frequency.
In other prior-art systems, the marker is excited into oscillations by signal pulses, or bursts, generated at the marker""s resonant frequency by a transmitter. When an exciting pulse ends, the marker undergoes damped oscillations at its resonant frequency (i.e., the marker xe2x80x9crings downxe2x80x9d), and this response (ring down) signal is detected by a receiver. Accordingly, prior systems generally involve a single resonant frequency dictated by the entire tag structure, and a uniform bias field. Multibit information encoded spatially cannot be recovered based on frequency response.
Indeed, an important advantage of the invention derives not only from the ability to obtain multiple resonances, but from the more general relationship between the amount of information that can be encoded on a tag and its physical complexity. If, for example, the amount of encodable information is determined by the number of resonant frequencies a tag exhibits, then the tag""s information-bearing capacity will grow much faster than its physical complexity, since each additional resonance requires only modest additional tag features. This means that linear increases in complexity (and, therefore, difficulty of fabrication) will produce substantially larger (e.g., exponential) increases in information-bearing capacity, rendering the present invention highly scalable and efficient.
In a second aspect, the invention comprises an information-bearing structure having multiple, non-equivalent pathways for electrical current. Each pathway encodes information recoverable by means of a wireless electromagnetic signal. Such a structure may, for example, take the form of a pair of conductive loops with one or more conductive crossbars extending thereacross, or a pair of matched conductive patterns forming one or more broken loops.